Multiple-Input Multiple-Output (MIMO) transmissions have recently been introduced in many modern wireless communication systems because they allow further increased data rates. For example, in the standardization committee 3GPP TS (3rd Generation Partnership Project Technical Standardization) a further development of UMTS (Universal Mobile Telecommunications System) is specified, named Long Term Evolution (LTE), which uses MIMO transmissions.
MIMO systems are characterized in that at each the transmitter side and the receiver side several antennas are provided for transmitting and/or receiving the signals. This allows the use of the spatial dimension for transmitting information, thus a higher spectral efficiency and higher data rates are possible without any increase in bandwidth.
Multi-user (MU) MIMO transmission systems allow the simultaneous transmission of different data streams to various users (receivers) on the same resource, i.e., for example, on the same frequency and/or at the same time. This may be achieved by a spatial multiplexing in the transmitter. In spatial multiplexing of signals for several users, the information symbols are pre-coded prior to their transmission, in order to multiplex the information in the spatial domain. The pre-coding used for a receiver will be signaled by the transmitter to the respective receiver so that the (receiver) is capable of detecting the signal pre-coded for it. While high requirements must be set regarding the transmitted different data streams being uncorrelated for a single-user (SU) MIMO transmission, in which several such pre-coded data streams are transmitted to a single user due to the proximity of the receiver antennas, the MU-MIMO transmission profits from the natural independence of the signals obtained at antennas of different receivers (and thus distanced from each other).
In spite of the spatial multiplexing of the different signals transmitted via the same resource, the interference by the other multiplexed signal or signals represents a considerable degree of disturbance for the signal to be detected by the receiver in question. By a suitable selection of the pre-coding vectors in the transmitter for the different receivers, the interference caused by the spatial multiplexing can be reduced, but it remains significant and leads to loss of performance in the receiver.
One option to suppress interferences caused for other users by the signals spatially multiplexed on the same resource comprises detecting these signals in the receiver and thus utilizing the deterministic nature of the interfering signals, i.e., considering them different from white noise. Such receivers are also called IA (Interference Aware) receivers. IA-receivers, such as, for example, IRC (Interference Rejection Combiners) and MMSE (Minimum Mean Square Error) receivers, are therefore particularly well suited for MU-MIMO transmission systems.
In the past, however, IRC and MMSE receivers required a perfect awareness of the interfering channel occurring at the receiver in question, via which the interfering signals are detected (i.e., the spatially multiplexed signal(s) for the other user(s) via the same resource or resources). This interfering channel comprises the MIMO channel for the receiver in question and the pre-coding vector or vectors for the mobile stations operated on the same resource. The MIMO channel of the receiver in question is continuously estimated thereby for the purpose of its own signal detection. The pre-coding vector or vector(s) for the other mobile stations are commonly not communicated by the base station (e.g., eNodeB) to the receiver in question (i.e., the receiver in the mobile station in question). Consequently, a mobile station is not or not entirely aware of the pre-coding vector or vectors for the other mobile stations operated on the same resource. This leads to the consequence that comprehensive information required for the use of IRC or MMSE receivers regarding the interfering channel is not provided at the receiver in question.